The invention relates to the fields of microfluidics, micro-total-analysis systems (μTAS) and micro-electro-mechanical systems (MEMS), in particular microfluidic pumps and mixers driven by induced-charge electro-osmosis.
The ability to transport fluids in micron-sized channels is essential for many emerging technologies, such as in vivo drug delivery devices, micro-electro-mechanical systems (MEMS), and micro-total-analysis systems (μTAS). New methods for the rapid mixing of non-homogeneous fluids in micron-scale devices are also required, since the absence of turbulent mixing on these small length scales implies that mixing occurs by molecular diffusion alone. This typically takes from seconds to minutes—far too slow for envisioned applications. New technologies are thus required for the manipulation, transport and mixing of fluids on these small length scales.
Although MEMS-based mechanical pumps with moving parts have recently been developed, including peristaltic pumps, a variety of non-mechanical pumping strategies without moving parts have been used, e.g. based on electrical fields, thermal gradient, electrochemical reactions, surface-tensions gradients, and patterned surfaces. Non-mechanical strategies for fluid manipulation become more efficient at very small scales because they are driven by surface phenomena. Moreover, they can be much cheaper to implement than mechanical MEMS-based strategies because they take advantage of nano-scale chemical effects already exhibited by many fluids used in biomedical and chemical engineering applications. They can also possess fewer parts, and are better suited for flexible devices, such as microfluidic fibers.
A popular non-mechanical fluid manipulation strategy is based on the phenomena of electro-osmosis, i.e. the fluid slip at a solid-electrolyte interface induced by a tangential electric field. The fluid is set into motion by strong electrostatic body forces exerted by excess ionic charge in diffuse boundary layers of thickness λ=1-100 nm near a solid interface. This effect, which has been studied extensively for more than a century in colloidal science and electrochemistry, is well suited for biomedical applications because the majority of bodily fluids, such as blood or lymph, are electrolytes with comparable ionic strengths. Moreover, the working electrode imposing spatially or temporally varying electric fields can be easily and cheaply built into microchannels with existing silicon-based micro-fabrication technology Driving fluids with electric fields also facilitates integration with logic circuits for sensing and integration microfluidic devices.
The simplest electro-osmotic pumping technique is based on applying a DC field tangential to a solid channel surface, presumed to have a uniform equilibrium zeta potential ζ or diffuse charge density q. In this case, the fluid-solid surface develops a ‘slip velocity’ given by the classical Helmlioltz-Smoluchowski formula defined as:
                                          μ            1                    →                =                                            (                                                                    -                    ɛ                                    ⁢                                                                          ⁢                                      ɛ                    o                                    ⁢                  ζ                                η                            )                        ⁢                                          E                                            →                                =                                    (                                                q                  ⁢                                                                          ⁢                  λ                                η                            )                        ⁢                                          E                                            →                                                          EQ        ⁢                                  ⁢        1            with a prescribed ζ or q, where δ0 is the permissivity of vacuum, and E and η represent the dielectric constant and viscosity of the electrolytic fluid.
In spite of its appealing simplicity, however, there are several drawbacks to the use of DC electric fields, related to the fact that a steady current (J=σE) must exit in order to maintain a steady field because every electrolyte has a non-negligible bulk conductivity. A steady current in turn implies the creation of ions at one electrode and removal of ions at the other via electrochemical reactions. This can cause a variety of problems. For example, the dissolution of the anode eventually destroys the electric circuit, causing irreversible failure. Microfluidic devices employing DC electric fields thus typically have short lifetimes, which can be acceptable in some applications, such as one-time drug delivery, but not in others, such as μTAS. A shorter lifetime also translates into a higher cost per unit of time of operation. The dissolution of the anode also injects metallic ions into the fluid, which can present safety hazards in biomedical applications or can interfere with chemical reactions in μTAS. Also, the depositions of ions at the cathode can lead to unstable deposits, which can break off or otherwise interfere with the bulk fluid. Furthermore, electrochemical reactions at electrodes inevitably cause electrolyte concentration gradients, which create complicated and potentially unwanted secondary bulk electric fields, as well as secondary electrokinetic phenomena at surfaces.
These problems can be solved using high-frequency AC fields, which can be safer, more reliable and more durable than using DC fields. Because AC fields are typically applied along closely spaced electrode arrays, much smaller voltages are required to achieve strong electric fields. Furthermore, the change in electrode polarity frustrates electrochemical reactions, helping avoid unwanted electrolysis reactions at the electrodes.
Since the fluid slip velocity of standard electro-osmosis used in EQ. 1 is linear in the applied field E, it averages to zero in an AC field. Therefore, different phenomena must be used is to drive steady microfluidic flows using AC fields. For example, AC traveling waves on electrode arrays have been used to drive flows by coupling to thermal gradients. A pair of electrodes adjacently located on a glass slide, to which an AC voltage is applied, has recently been shown to drive a steady swirling flow, and a stationary AC wave on a locally asymmetric electrode array has been shown to pump fluid. Both of these applications work in a limited range of frequencies and rely on a subtle form of electro-osmosis involving induced charges on the electrodes. The electro-osmotic flow is driven by transient interactions between the high-frequency field and the self-induced changes in the diffuse-layer charge density along the electrode surfaces. The pumping effect is therefore a strictly non-equilibrium phenomenon which violates the ubiquitous assumption of a constant zeta potential underlying the classical theory of electro-osmosis.
Although the available pumping techniques based on AC electric fields offer various advantages over DC methods, there are still serious drawbacks. Foremost among these is the need to microfabricate complex patterned-surface electrodes with elaborate micro-circuitry, which can be more costly, difficult, and prone to failure than their very simple DC counterparts. Another potential drawback is that patterned-surface devices are “hard-wired” into the electrical circuitry and the physical structure of the surface itself, rendering them less versatile.